A variety of processes have been marketed and/or proposed over the years for welding or bonding together overlapping portions of a tensioned loop of thermoplastic strap encircling an article. In the context of this subject matter, the terms "joint," "weld," and "welded joint" are conventionally used interchangeably to designate the bonded or joined-together portions of the strap.
In one such process, a hot blade is employed to melt the interface portions of the overlapping strap lengths which then resolidify to form the weld. However, this process typically generates vapor and smoke which may be objectionable. Thus, it would be desirable to provide an improved process wherein smoke generation is eliminated or substantially reduced.
Another process is effected by first pressing the overlapping strap portions together and then creating a unidirectional or multidirectional bodily sliding frictional movement between the contacting surfaces of the overlapping strap portions so as to melt the interface region of the overlapping strap portions. The melted interface region is allowed to solidify at rest, either under pressure or free of pressure, so as to bond the overlapping strap portions together.
This process, which can be generally designated as "friction-fusion welding" or "friction welding", has proven to be especially effective with conventional thermoplastic strap materials such as nylon, polyester, and polypropylene. Such conventional strap is typically provided commercially in widths ranging from about 5 mm. to about 13 mm. and in thicknesses ranging between about 0.25 mm. and about 0.89 mm. Some systems for making a friction-fusion weld between portions of such strap employ high-speed, reciprocating mechanisms, and these can produce considerable noise during operation. It would be desirable to provide an improved system which would produce an effective friction-fusion weld with substantially less noise.
Welded joints in thermoplastic strap have found wide commercial acceptance in many applications. However, a welded joint is typically the weakest part of a tensioned strap loop secured about a package or other object. Attempts have been made to produce stronger welds.
A unique strap joint or weld having greater strength, and methods for producing such welds, are disclosed in U.S. Pat. Nos. 4,707,390 and 4,776,905. The weld is formed by first fusing (melting or liquefying under the application of heat) at least part of the thickness of each strap portion across the width of the interface. The fused parts of the strap portions are then permitted to subsequently solidify to form the welded joint. The weld is created with a selected strap material in a manner such that cavities (such as voids, bubbles, or discrete volumes of a separate material) are encapsulated within the resolidified region of the weld. The cavities have been found to result in the weld having a greater strength as discussed hereinafter.
The cavities are dispersed generally across the width of the strap weld and are preferably more generally concentrated toward each longitudinal end of the weld. In the preferred form of the weld, the concentration of cavities in the weld middle portion is substantially less than at the weld ends.
In one form of the method for producing the improved joint of the invention, the cavities are believed to result from the production of gaseous bubbles during the welding process. It is believed that some types of strap contain significant amounts of an additional material, such as moisture, which can exist as a gas when the strap portions are melted under pressure. For example, polyester and polyamide nylon strap are hygroscopic and can contain some amount of water.
It is theorized in U.S. Pat. No. 4,707,390 that when portions of such strap are melted and are under pressure during welding, the generated gas bubbles tend to be forced outwardly toward the edges and ends of the weld. If the welding pressure is terminated while the strap portions are still molten and before all of the bubbles have been squeezed out of the weld area, the remaining bubbles become encapsulated within the solidifying strap material to form the cavities.
The cavities may be defined by other structures and materials which are provided as part of the strap structure as disclosed in the U.S. Pat. No. 4,892,768. In particular, the strap can include (1) at least a first layer of a first material that does not produce the desired cavities, and (2) a second material for effecting the creation of the plurality of discreet volumes in the resolidified region as a result of application of heat and subsequent resolidification.
In one embodiment, the second material is present on at least one side surface of the first material layer, and the second material defines a second layer carried on the first material layer. An example of the first material is polypropylene or polyethylene terephthalate. An example of the second material is polyethylene terephthalate having an intrinsic viscosity of about 1.0.
In another embodiment, the second material is dispersed as an additive in the first material layer and has a greater concentration at least at one side surface of the first material layer. The additive second material may be a foaming agent material such as a 5-phenyltetrazole compound or a toluenesulfonyl semicarbicide compound.
In another embodiment, the second material may disperse to form separate globules which occupy the discreet volumes. Such a second material may be the combination of polyisobutylene dispersed in a layer of polypropylene which in turn is carried on the first material layer.
With appropriate welding techniques, the above-described strap compositions can produce welded joints with cavities distributed in the weld, including in the outer or end portions of the weld. The above-referenced U.S. Pat. Nos. 4,707,390 and 4,892,768 each disclose that the exact mechanism by which the cavities increase the weld strength is not necessarily fully or accurately understood, but nevertheless propose a theory. In particular, it is believed that the improved joint strength of the weld results from the redistribution of stresses within the weld, and that the cavities cause the stress redistribution. It is believed that welds tend to fail when cracks form at the ends of the weld. The cavities are believed to reduce the stresses at a crack tip that is propagating into one or more of the cavities.
While the above-described type of cavity-containing weld provides increased strength and functions well in a variety of applications, there is a continuing need for a system that can produce high quality welds routinely and consistently, and in an economic manner.
In particular, it has been found in some applications that, depending on the strap material, strap size, and welding parameters, the capability for repeatedly producing acceptable welds having a desired high strength is not as good as would be desired. It has been theorized that in some such cases, the cavities may not be distributed in a manner that provides the desired level of increased joint strength. For example, there may be too many cavities in the central portion of the weld. The central portion of the weld may then have an excessively "foamed" structure while the peripheral region of the weld has too few cavities. It is suspected that, in some situations, this might actually decrease the strength of the weld. Accordingly, it would be desirable to provide a system for better controlling the creation and distribution of cavities in the weld.
The present invention provides an improved welded joint between overlapping strap lengths and also provides an apparatus and method which can accommodate designs having the above-discussed benefits and features.